Method for catalytic and selective oxidation of aromatic compounds

ABSTRACT

The invention relates to the use of compounds of the formula 
     
       
         R 1   a Re b O c .L d   (I), 
       
     
     in which                    
     and the total of a, b and c is such as to comply with the pentavalency or heptavalency of rhenium, with the proviso that c is not larger than 3×b, and in which R 1  is absent or identical or different, and is an aliphatic hydrocarbon radical having 1 to 10 carbon atoms, an aromatic hydrocarbon radical having 6 to 10 carbon atoms or an arylalkyl radical having 7 to 9 carbon atoms, it being possible for the R 1  radicals where appropriate to be substituted identically or differently, independently of one another, as catalysts for oxidizing electron-rich aromatic compounds and their derivatives, the catalysts being employed in a peroxide-containing solution in the presence of an anhydride of a carboxylic acid and/or of a dehydrating agent.

The present invention relates to the use of Re catalyst systems for theoxidation of electron-rich aromatic compounds, the catalyst systemhaving not only an extended useful life but also excellent activity, andto a process for the oxidation of electron-rich aromatic compounds usingthis catalyst system.

Oxidative processes play a crucial part in organic synthesis. Numerousbasic and fine chemicals are prepared using atmospheric oxygen, hydrogenperoxide and alkyl peroxides. In most cases, the reactions are efficientand selective only if the oxidizing agents are used in the presence ofcatalysts. In practice, these are mostly metal oxides such as, forexample, V₂O₅, CrO₃, MoO₃, WO₃, OsO₄ and RuO₄, which are employed, forexample, in epoxidation, hydroxylation or carboxylation reactions(Catalytic Oxidations with Hydrogen Peroxide as Oxidant (Ed.: G.Strukul), Kiuwer, Dordrecht, 1992, 13-43; H. A. Jørgensen, Chem. Rev.1989, Vol. 98, pp. 431-458).

Application of such systems to the catalytic oxidation of aromaticcompounds is not self-evident: lack of activity (WO₃) on the one hand,deficient selectivity on the other hand (CrO₃/H₂SO₄), tendency tocatalytic decomposition of hydrogen peroxide (RuCl₃), besides thefrequent lack of acceptability in ecological and health terms (forexample in the case of OsO₄ or CrO₃), which result in complicated andcostly disposal, have to date prevented the use of such catalysts.

Other processes already established in oxidation chemistry andemploying, for example, electrochemical oxidation, cerium(IV) salts,manganese(III) sulfate or peroxides (tBuOOH) in the presence ofmolybdenum complexes as oxidizing agents prove in the oxidation ofsimple or fused aromatic compounds and their derivatives to becomplicated and costly, and are often associated, owing to the need toemploy stoichiometric amounts (cerium(IV) salts, manganese(III)sulfate), with high salt burdens and are usually also nonspecific (R. P.Kreh et al., J. Org. Chem., 1989, 54, 1526-1531; M. Hudlicky, Oxidationsin Organic Chemistry, ACS Monograph 186, Washington D.C. 1990, pp.92-98; T. A. Gorodetskaya et al. U.S.S.R. Patent 1 121 255, 1984; Chem.Abstr., 1985, 102, 203754; W. Adam et. al., Synthesis, 1993, 280-282, J.Skarzewski, Tetrahedron, 1984, 40, 4997-5000; S. Yamaguchi et al., Bull.Chem. Soc. Jpn. 1986, 59, 2881-2884; M. Perisamy, M. V. Bhatt,Tetrahedron Lett. 1978, 4561-4562, Y. Asakawa et al., 1988, J. Org.Chem., 53, 5452-5457; W. Chen, Chem. Abstr., 1987, 107, 58620).

EP 0665209 A1 and EP 0686618 A1 disclose organorhenium compounds of thegeneral type R¹ _(a)Re_(b)O_(c).L_(d) (L=Lewis base) which are employedas catalysts for oxidizing a large number of aromatic compounds toquinoid systems in the presence of hydrogen peroxide.

These rhenium compounds can be synthesized in a simple manner fromcommercially available Re₂O₇ with the aid of conventional substanceswhich act as donors of organic groups, for example in the case of R¹=CH₃by reaction with commercially available tetramethyltin or commerciallyavailable dimethylzinc. They are insensitive to air and moisture, can bestored at room temperature and are, in conjunction withperoxide-containing compounds such as, for example, hydrogen peroxide,suitable catalysts for oxidizing aromatic compounds. However, it hasemerged that the useful life of the catalysts, especially at theelevated temperature which is normally necessary, is only inadequate.

The object therefore was to find a catalyst system which is, wherepossible, easily obtainable, low-cost, simple to handle, storable andeffective and which, besides high selectivity in the oxidation ofaromatic compounds, shows improved useful life of the catalyst andexcellent activity.

It has now been found, surprisingly, that on addition of anhydrides ofcarboxylic acids to the reaction system in combination withperoxide-containing compounds there is a drastic increase in thecatalytic activity of rhenium compounds. This results in a decisiveimprovement in the previously disclosed process and was by no means tobe expected.

The invention therefore relates to the use of compounds of the formula

R¹ _(a)Re_(b)O_(c).L_(d)  (I),

in which

and the total of a, b and c is such as to comply with the pentavalencyor heptavalency of rhenium, with the proviso that c is not larger than3×b, and in which R¹ is absent or identical or different, and is analiphatic hydrocarbon radical having 1 to 20 and preferably from 1 to 10carbon atoms, an aromatic hydrocarbon radical having 6 to 20 andpreferably from 6 to 10 carbon atoms or an arylalkyl radical having 7 to20 and preferably from 7 to 9 carbon atoms, it being possible for the R¹radicals where appropriate to be substituted identically or differently,independently of one another, and in the case of δ-bonded radicals atleast one hydrogen atom is still bonded to the carbon atom in the αposition, as catalysts for oxidizing electron-rich aromatic compoundsand their derivatives in the presence of an anhydride of a carboxylicacid and of a peroxide-containing compound.

The invention further relates to a process for the oxidation ofelectron-rich aromatic compounds, which comprises oxidizingelectron-rich C₆-C₂₂-aryl compounds and their derivatives in thepresence of a catalyst of the formula R¹ _(a)Re_(b)O_(c).L_(d) (I), inwhich R¹, a, b, c, d and L have the abovementioned meaning, of aperoxide-containing compound and of an anhydride of a carboxylic acid.

The compounds of the formula (I) can also be in the form of their Lewisbase adducts. Typical examples of Lewis bases are pyridine, bipyridine,t-butylpyridine, amines, in particular secondary and tertiary aminessuch as triethylamine and quinuclidine, H₂O and polyethers such as, forexample, diglyme.

An aliphatic hydrocarbon radical R¹ means alkyl radicals having 1 to 20and preferably from 1 to 10 carbon atoms, alkenyl or alkynyl radicalshaving 2 to 20 and preferably from 2 to 10 carbon atoms, cycloalkyl orcycloalkenyl radicals having 3 to 20 and preferably from 3 to 10 carbonatoms. Suitable examples of R¹ are alkyl radicals such as methyl, ethyl,propyl, isopropyl and the various butyl, pentyl, hexyl, octyl radicalssuch as ethylhexyl and decyl radicals, and alkenyl radicals such asallyl; also suitable are cycloalkyl radicals such as cyclopropyl,cyclobutyl, cyclopentyl, alkylated cyclohexyl such as hydrogenatedtolyl, xylyl, ethylphenyl, cumyl or cymyl, 1-menthyl and 1-norbonyl, andalkenyl radicals such as vinyl and allyl and cycloalkenyl radicals suchas cyclopentadienyl and pentamethylcyclopentadienyl.

Suitable examples of an aromatic hydrocarbon radical R¹ are phenyl ornaphthyl. Benzyl may be mentioned as an example of an arylalkyl radical.

The radical R¹ can also be substituted. Examples of suitablesubstituents are fluorine, chlorine, bromine, NH₂, NR² ₂, PH₂, PHR², PR²₂, OH or OR², where R² is identical or different and is an alkyl radicalhaving 1 to 20 and preferably from 1 to 10 carbon atoms or an arylradical having 6 to 20 and preferably from 6 to 10 carbon atoms, whichmay, for example, have the meanings stated above for R¹.

Very particularly preferred compounds of the formula (I) are the rheniumoxides methylrhenium trioxide (CH₃ReO₃), cyclopentadienylrheniumtrioxide (CpReO₃), cyclopropylrhenium trioxide (C₃H₅ReO₃) and dirheniumheptoxide (Re₂O₇).

It is essential to the invention that the reaction system comprises ananhydride of a carboxylic acid.

Particularly suitable anhydrides for the present invention are those ofaliphatic carboxylic acids, with anhydrides of aliphatic carboxylicacids having 1-6 carbon atoms being preferred, and it also beingpossible for the carboxylic acid to be unsaturated and/or branched.

Suitable examples are, in particular, the anhydrides of acetic acid,propionic acid, n-butyric acid, n-valeric acid, n-caproic acid,i-butyric acid, i-valeric acid, ethylmethylacetic acid, trimethylaceticacid, propenoic acid, methacrylic acid, crotonic acid and vinylaceticacid.

The amount of anhydride to be added is not critical but a mixing ratioof anhydride to solvent of from 1:10 to 10:1 has proven suitable.

It is possible to use organic solvents as liquid medium for the reactionsystem.

Examples of suitable solvents are the anhydride itself, a carboxylicester such as ethyl acetate, a carboxylic acid, alcohols having 1-5carbon atoms such as methanol, ethanol and the various propanols andbutanols, with tert-butanol being particularly preferred, aromatichydrocarbons such as toluene, ethers such as diethyl ether, di-n-butylether, tert-butyl methyl ether, aliphatic hydrocarbons such as hexane,heptane, methylene chloride, tetrahydrofuran and acetonitrile, andmixtures thereof.

It is preferred according to the invention to use a carboxylic acid, inparticular the carboxylic acid corresponding to the anhydride.

It is preferred to use a combination of carboxylic acids and theiranhydrides of the formula (II)

R_(a) COOH

and

R_(a)C(O)O(O)CR_(a)  (II)

where R_(a) is an aliphatic hydrocarbon radical having 1 to 10 andpreferably from 1 to 5 carbon atoms.

Examples of suitable representatives are the same as those mentionedabove.

Because it is easily available, acetic acid (glacial acetic acid) andits anhydride are particularly preferred.

The added anhydride may in this case perform several functions: it mayon the one hand act as precursor for peracid, and on the other hand asdehydrating agent to trap water from the reaction solution. Thehydrolysis product is moreover, in the preferred case, identical to thecarboxylic acid employed and facilitates subsequent working up of thereaction solution, because it is unnecessary to employ anysalt-containing and, in particular, halogen-containing desiccants whichmust subsequently be removed, possibly with difficulty, from theproduct.

The anhydride employed is, just like, for example, acetic acid andhydrogen peroxide, a low-cost bulk chemical and provides a distinctsaving in costs compared with the previous process, especially in viewof the fact that the content of costly rhenium-based catalysts can bereduced by 50-75%. In addition, besides the increase in reactivity,there is found to be an increase in the selectivity to as much as 92%based on the starting material employed.

It has emerged that the useful life of the catalysts can be increasedfurther in particular under conditions which are as anhydrous aspossible.

In another embodiment of the invention, the water of reaction can alsobe removed from the reaction system by adding dehydrating agents. It isthen possible in this case to dispense with the use of the anhydride.However, it is also possible to employ the anhydride and the dehydratingagent together. Conventional organic and inorganic dehydrating agentscan be used for the present invention as long as they are compatiblewith the peroxide-containing compound.

Examples thereof are MgSO₄, Na₂SO₄, CaCl₂, H₂SO₄ and orthoesters ofcarboxylic acids.

The amount of dehydrating agent to be added depends on itswater-absorbing capacity and is in general from 0.05 to 10 moleequivalents based on the amount of peroxide-containing compound.

Aryl compounds suitable for the process according to the invention areelectron-rich aromatic compounds or fused aromatic systems having 6 to22 carbon atoms, preferably having 6 to 14 carbon atoms, which may,where appropriate, be substituted one or more times, identically ordifferently by an electron donor group. Typical suitable electron donorgroups are hydroxyl, C₁-C₃-alkoxy, N-acylamino-,N-acylamino-C₁-C₃-alkyl, acyloxy and C₁-C₃-alkyl radicals.

Examples of such aryl compounds are xylenes, di-, tri- ortetrasubstituted C₁-C₃-alkylbenzenes or C₁-C₃-alkoxybenzenes, and, inparticular, naphthalene and its C₁-C₃-alkyl or C₁-C₃-alkoxy derivativessubstituted once to six times, anthracene and its C₁-C₃-alkyl orC₁-C₃-alkoxy derivatives, phenanthrene and more extensively fusedaromatic compounds, phenols, hydroquinone, resorcinol, catechol andpyrrogallol, but also biphenyl.

Preferred aryl compounds are naphthalene and anthracene, and theirderivatives, and naphthalene and its derivatives are particularlypreferred, especially 2-methylnapthalene.

The process according to the invention generally results in the arylcompounds being oxidized to the corresponding quinoid systems. Forexample, 2-methylnaphthalene results in 2-methyl-1,4-naphthoquinone,which is the basis of the vitamin K series.

In the case of aryl compounds with more substituents (three or more),where the formation of a quinoid system is impossible, the processaccording to the invention results in preparation of the correspondinghydroxyl compound. Typical examples of such aryl compounds with moresubstituents are 1,2,3,5,8-pentamethylnaphthalene;1,2,3-trimethylbenzene, mesitylene and 1,3,5-trimethoxybenzene. Thesestarting materials afford by the process according to the invention, forexample, the following hydroxyl compounds:4-hydroxy-1,2,3,5,8-pentamethylnaphthalene and1-hydroxy-2,4,6-trimethoxybenzene.

Examples of suitable peroxide-containing compounds are hydrogenperoxide, inorganic peroxides, for example alkali metal peroxides suchas sodium peroxide, and percarboxylic acids and their salts such as, forexample, m-chlorobenzoic acid, peracetic acid and magnesiummonoperoxophthalate, with hydrogen peroxide being particularly preferredbecause of its ready availability.

In the process according to the invention, the aromatic compound to beoxidized is dissolved in the organic solvent mixture described; and thecatalyst is added.

The concentration of dissolved aromatic compound based on the solvent is0.1 mol/l-10 mol/l, preferably 0.4 mol/l-4 mol/l and particularlypreferably 0.5 mol/l-2 mol/l.

The catalyst can be employed in an amount of 0.01-10 mole percent,preferably 0.02-2 mole percent (Re metal catalyst based on the compoundto be oxidized). The peroxide-containing compound (5-90 percent byweight) is normally added to this solution in a molar ratio of from 1:1to 20:1 based on the compounds to be oxidized.

The reaction mixture is stirred at a temperature of, normally, 10-100°C., preferably 20-60° C., until conversion is complete. The reactionmixture is then worked up in a manner which is usual for the skilledworker, i.e. for example neutralized, extracted and dried. The crudeextracted product can be further purified, for example, by high vacuumdistillation or recrystallization.

The rhenium compounds of the formula (I) are compounds commerciallyobtainable by purchase (Re₂O₇) or can easily be prepared (G. Brauer,Handbuch der Präparativen Anorganischen Chemie [Handbook of preparativeinorganic chemistry], 3rd edition, Enke-Verlag, Stuttgart 1981, W. A.Herrmann, J. Organomet. Chem., 1995, 500, 149-174.).

Although their suitability in principle as oxidizing catalyst foraromatic compounds was known (EP 0665209 A1 and EP 0686618 A1),combining these catalysts with a peroxide-containing compound such as,for example, hydrogen peroxide and an anhydride of a carboxylic acidand/or from dehydrating agents is novel and results in a drasticincrease in the catalytic activity, which was by no means to be expectedbecause, in particular, the resistance to anhydrides of the rheniumcompound used is surprising according to the current state of knowledge.The described invention thus represents a considerable advance bycomparison with the previously disclosed processes, especially withregard to useful life, activity and selectivity, and is therefore ofconsiderable significance in practice.

The following examples serve to illustrate the present invention.

EXAMPLES

General Method for the Rhenium-catalyzed Oxidation of Aromatic Compounds

1.00 g (7 mmol) of 2-methylnaphthalene is dissolved in a combination of10 ml of glacial acetic acid and 5 ml of acetic anhydride in a reactionvessel maintained at the required temperature, and 0.5 to 1.0 molepercent of the appropriate catalyst is added. Finally, 1.4 ml of H₂O₂(85 percent by weight) are added (aryl compound/H₂O₂ molar ratio: 1:7).The reaction is stirred at 40 or 60° C. for 4 h or longer (see table),ensuring exact maintenance of the temperature.

Workup

The reaction solution is neutralized with a saturated sodium bicarbonatesolution, the aqueous mother liquor is extracted with methylenechloride, and the combined substrates are dried over MgSO₄. The solventis removed in vacuo, resulting in 2-methyl-1,4-naphthoquinone as solidyellow oxidation product.

The method can be applied analogously to other aromatic compounds.

The examples carried out in accordance with the above method are to befound in Table 1.

Control experiments were carried out likewise in accordance with theabove method but without addition of catalyst. Conversion at 40° C.after 4 h was only 13% at 40° C. and 60% at 60° C. with a selectivity of17 and 25%, respectively, based on 2-methylnaphthalene (startingmaterial).

TABLE 1 Compilation of oxidation examples Concentration T t ConversionSelectivity^([a]) No. Catalyst [mol %] [° C.] [h] [%] [%]  1 MTO 0.5 401.3 46 55  2 MTO 1.0 40 4.0 74 54  3 CpReO₃ 0.5 40 4.0 60 66  4 CpReO₃1.0 40 4.9 71 68  5 MTO 0.5 60 3.5 89 75  6 MTO 1.0 60 2.5 87 92  7CpReO₃ 0.5 60 1.5 75 85  8^([b]) MTO 2.0 40 4.0 56 61  9^([b]) — — 404.0 13 17 10^([b]) — — 60 4.0 60 25 MTO = Methyltrioxorhenium CpReO₃ =Cyclopentadienyltrioxyrhenium Solvent Nos. 1-8 = 10 ml of glaciai aceticacid plus 5 ml of acetic anhydride

[a] The selectivity is based on 2-methylnaphthalene (starting material)

[b] Control experiments:

Solvent No. 8 15 ml of glacial acetic acid (reference: W. Adam et al.,Angew. Chem. 1994, 33, 2475-2476).

Solvent Nos. 9 and 10 Experiments carried out in accordance with theabove method in the absence of catalyst

Example 11 and Comparative Example 12

Oxidation of 2,3,5-trimethylbenzene (TMB) with and without Addition ofDehydrating Agent

Composition of the Reaction Solution

50 mg MTO (0.2 mmol, 2 mol %)

1,2 g TMB (10 mmol)

2 ml H₂O₂ (85%, ca. 70 mmol)

in 20 ml acetic acid.

Working up was carried out by extraction with dichloromethane, washingwith water, stripping off the solvent and purifying the resultingproduct on a silica gel column (pentane/EtOAc 5:1).

The following yields are obtained:

2 h/70° C.: 8%

2 h/70° C. and addition of 1.5 g of MgSO₄: 16%

4 h/40° C.: 8%

4 h/40° C. and addition of 1.5 g of MgSO₄: 11%.

What is claimed is:
 1. A process for the oxidation of election-richaromatic compounds, which comprises oxidizing one or more election-richC₆-C₂₂-aryl compounds and their derivatives in a solution of ananhydride of a carboxylic acid and a carboxylic acid in the presence ofa catalyst, wherein the catalyst has the formula: R¹_(a)Re_(b)O_(c).L_(d)  (I), in which

and the total of a, b and c is such as to comply with the pentavalencyor heptavalency of rhenium, with the proviso that c is not larger than3×b, with the exception of Re₂O₂, in which R¹ is absent or identical ordifferent, and is an aliphatic hydrocarbon radical having 1 to 10 carbonatoms, an aromatic hydrocarbon radical having 6 to 10 carbon atoms or anarylalkyl radical having 7 to 9 carbon atoms, wherein the R¹ radicals,where appropriate, are optionally substituted identically ordifferently, independently of one another, and in the case of δ-bondedradicals, at least one hydrogen atom is still bonded to the carbon atomin the α-position and wherein, the anhydride is of the formulaR_(a)C(O)O(O)CR_(a) and the carboxylic acid is of the formula R_(a)COOHwhere R_(a) is an aliphatic hydrocarbon radical having 1 to 10 carbonatoms, and the mixing ratio of anhydride to carboxylic acid is from 1:10to 10:1.
 2. The process according to claim 1, where the catalyst is acompound of formula (I) wherein R¹ is a C₁-C₃ alkyl a is 1, b is 1, andc is
 1. 3. The process according to claim 1, where the catalyst is acompound of formula (I) wherein R¹ is cyclopentadienyl or its methylderivative a is 1, b is 1, c is 1, and d is O.
 4. The process accordingto claim 1, wherein the peroxide-containing compound is hydrogenperoxide.
 5. The process according to claim 1, wherein the electron-richaryl compound is a C₆-C₂₄ aryl compound.
 6. The process according toclaim 1, wherein the electron-rich aryl compound is naphthalene or itsderivatives.
 7. The process according to claim 6, wherein theelectron-rich aryl compound is a 2-methylnaphthaline.
 8. The processaccording to claim 1, wherein the anhydride is acetic anhydride and thecarboxylic acid is glacial acetic acid.
 9. The process according toclaim 1, wherein the oxidation is carried out at a temperature selectedfrom the range of 10 to 100° C.
 10. The process according to claim 1which further comprises a dehydrating agent.
 11. The process accordingto claim 10 wherein the dehydrating agent is MgSO₄, Na₂SO₃, CaCl₂, H₂SO,or an orthoester of a carboxylic acid.